I. Field of the Invention
The present invention relates to wireless communications. More specifically, the present invention relates to wireless communication systems that transmit and receive signals between base stations and mobile stations in accordance with code division multiple access (CDMA) modulation. Even more specifically, the present invention relates to a method and apparatus for reducing co-channel interference in a CDMA system in cases where the same user is simultaneously transmitting data on multiple CDMA channels in order to, for example, achieve a high data rate.
II. Description of the Related Art
In multi-code (MC) CDMA radio telephone systems, several CDMA signals associated with a single user may be sent between two stations in order to provide the user with a higher data rate. In such systems, each CDMA signal associated with a single user is spread using direct sequence (DS) spreading and orthogonal Walsh codes, and the spread signals are scrambled using pseudo-noise (PN). However, the transmission quality of the CDMA signals in such systems typically degrades when the signals travel over more than one path between the transmitter and the receiver. This is because such xe2x80x9cmulti-path propagationxe2x80x9d causes co-channel interference between the transmitted CDMA signals. The present invention is directed to a method and apparatus for reducing (or canceling) such co-channel interference.
Disclosed herein is a method and apparatus for reducing co-channel interference in a CDMA radio telephone system. The disclosed system transmits a traffic channel signal from a first station (e.g., a base station) to a second station (e.g., a mobile station). The traffic channel signal contains a plurality of CDMA data signals, each data signal modulated with a different Walsh code prior to transmission of the traffic channel signal from a first station to a second station. In one embodiment of the disclosed system, the traffic channel signal contains a plurality of CDMA data signals associated with a single user. Each of the CDMA data signals associated with the single user is modulated with a different Walsh code prior to transmission of the traffic channel signal from the first station to the second station. Use of the plurality of CDMA data signals for the single user facilitates transmission of data at a relatively high data rate for the single user, because such signals permit the simultaneous transmission of data associated with the single user on multiple CDMA channels.
In accordance with the preferred embodiment of the disclosed method and apparatus, a xe2x80x9creduced interference signalxe2x80x9d is formed at the second station in an iterative fashion by subtracting an interference signal from the traffic channel signal. A xe2x80x9cdecoded signalxe2x80x9d is formed at the second station by decoding information represented by the reduced interference signal using a decoder (e.g., a convolutional or turbo decoder) that outputs a plurality of metric values. It should be noted that the first iteration has no correction. The metric values are then used as inputs to a feedback loop that is used to form the interference signal that is subtracted from the traffic channel signal. In this feedback loop: (1) an estimated signal is formed in accordance with the plurality of metric values; (2) plurality of reconstructed CDMA signals are formed from the estimated signal; (3) each of the reconstructed CDMA signals being modulated in accordance with a different one of the Walsh codes; and (4) the reconstructed CDMA signals are combined, scrambled and filtered to form the interference signal.